System for estimating cardiac pressure using parameters derived from impedance signals detected by an implantable medical device

ABSTRACT

A method of monitoring pressure within a medical patient, includes measuring an actual pressure in a medical patient in a first time period; measuring an indicator of the actual pressure in the first time period, wherein the indicator is derived from an electrical signal of the patient&#39;s heart; determining a correlative relationship between the actual pressure and the indicator, wherein both the actual pressure and the indicator are obtained in the first time period; measuring the indicator in a second time period; and determining the actual pressure in the second time period based on the correlative relationship obtained in the first time period and the indicator obtained in the second time period.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/520,454, filed Sep. 13, 2006, which claims priority from U.S.Provisional No. 60/716,611, filed Sep. 13, 2005, and which is acontinuation-in-part of U.S. application Ser. No. 10/698,031, filed Oct.29, 2003, which claims priority from U.S. Provisional No. 60/470,468,filed May 13, 2003 and which is a continuation-in-part of U.S.application Ser. No. 10/127,227, filed Apr. 19, 2002, which is acontinuation of U.S. application Ser. No. 09/956,596, filed Sep. 19,2001, now abandoned, which is a continuation of U.S. application Ser.No. 09/481,084, filed Jan. 11, 2000, now U.S. Pat. No. 6,328,699, all ofwhich are hereby incorporated by reference.

SUMMARY

In one embodiment, the invention comprises a method of monitoringpressure in an individual. In one embodiment, the method comprisesmeasuring a true or actual pressure in a patient, and measuring one ormore indicators of that pressure. In some embodiments, the actualpressure may be difficult to assess or require invasive means. Thus,using an indicator of actual pressure (which may be easier to determineor access) may facilitate determination of actual pressure. Bycorrelating the indicators of actual pressure with the actual pressureduring a first given time period, an actual pressure determination in asecond given time period can be made by relying solely on the indicatormeasurement. In this way, actual pressure can be measured in aless-invasive and facile manner.

In other embodiments, a pressure sensor is implanted in a patient. Thepressure sensor may be susceptible to drift or inaccuracy over time. Bycorrelating the actual pressure (as determined by the implanted pressuresensor) and one or more indicators of pressure, a correlativerelationship (or pre-determined relationship) between the actualpressure and the indicator values may be established. Accordingly, thiscorrelation can be used at a later time to determine if the pressuresensor requires calibration. Alternatively, this correlation can be usedto determine actual pressure, even without calibration. The pressuresensor may include the data processing system associated with thesensor.

In one embodiment, a method of monitoring pressure within a medicalpatient is provided. In one embodiment, the method comprises: measuringan actual pressure in a medical patient in a first time period;measuring an indicator of the actual pressure in the first time period,wherein the indicator is derived from an electrical signal of thepatient's heart; determining a correlative relationship between theactual pressure and the indicator, wherein both the actual pressure andthe indicator are obtained in the first time period; measuring theindicator in a second time period; and determining the actual pressurein the second time period based on the correlative relationship obtainedin the first time period and the indicator obtained in the second timeperiod.

The first time period and second time periods can include any intervalof time. Any interval may be selected, such as minutes, hours, days,weeks, months, and/or years, or a combination thereof. Thus, in oneembodiment, data regarding actual pressure and indicator value areobtained in a first time period for 1-20 weeks to establish acorrelative relationship. The correlation is used in a second timeperiod, which can include the following hours, days, weeks, months oryears, after the first time period to calculate an actual pressure basedsolely on one or more indicator values obtained during the second timeperiod. Alternatively, the indicator data obtained in the second timeperiod may be used to calibrate a pressure sensing device, wherein saidcalibration can occur automatically or via patient instructions.

The data obtained in the first time period may be considered historicaldata, such as historical actual pressure data and historical indicatordata. Data may include calculations and measurements obtained from onepatient or a sampling of several patients. For example, in oneembodiment a patient's actual pressure in the second time period isdetermined based on the relationship between the actual pressure dataand indicator data in the first period, wherein that first period datais historical data obtained from one or more other patients. In anotherembodiment, the historical data is the patient's own data.

The indicator can be selected from a group consisting of one or more ofthe following: an a-wave, a v-wave, an x-descent, a y-descent, anechocardiogram waveform, and an IEGM waveform. The correlativerelationship can be based at least in part on at least one of anamplitude and a timing parameter of the indicator, wherein the indicatoris an electrocardiogram signal. In addition, the correlativerelationship can comprise an artificial neural network. The determiningstep can be performed within the medical patient.

In another embodiment, a method of monitoring pressure within a medicalpatient, comprises: measuring an indicator of a pressure within amedical patient, wherein the indicator is related to an electricalsignal generated by a patient; determining the pressure based at leastin part upon the indicator and a pre-determined relationship between thepressure and the indicator, wherein the pre-determined relationship isdetermined from historical pressure data and historical indicator dataof the patient; and providing an indication of the pressure.

At least one of the historical pressure data and the historicalindicator data can include data from only the medical patent.

In another embodiment, a method of calibrating a pressure monitoringsystem, comprises: measuring an actual pressure within the heart of amedical patient with a pressure monitoring system; measuring anindicator of the actual pressure, wherein the indicator is related to aphysiological parameter of the medical patient; calculating a pressurevalue indicative of the actual pressure based at least in part upon theindicator of the actual pressure and a pre-determined relationshipbetween the actual pressure and the indicator of the actual pressure,wherein the pre-determined relationship is determined from historicalpressure data and historical indicator data of the patient; determiningan error value based upon the difference between the measured actualpressure and the calculated pressure value; and calibrating the pressuremonitoring system when the error value exceeds a predeterminedthreshold.

The pre-determined relationship can include an artificial neuralnetwork. In another embodiment, the actual pressure is a left atrialpressure of a medical patient. In one embodiment, calibrating thepressure monitoring system comprises subtracting the error value fromthe measured pressure value.

In another embodiment, the method further comprising generating an alertwhen the error value exceeds the predetermined threshold. In oneembodiment, generating an alert comprises providing an instruction tothe medical patient to perform a patient calibration procedure. Thepatient calibration procedure can include performing a Valsalvamaneuver. In one embodiment, the calculating is performed within themedical patient.

In yet another embodiment, a method of controlling delivery of a therapydelivered to a patient, comprises: measuring an indicator of an actualpressure within a medical patient, wherein the indicator corresponds toa physiological parameter of the medical patient; determining the actualpressure based at least in part upon the indicator and a pre-determinedrelationship between the actual pressure and the indicator, wherein thepre-determined relationship comprises a neural network trained withhistorical pressure data and historical indicator data; and controllinga therapy based upon the determined pressure.

In one embodiment, controlling a therapy comprises controlling at leastone device selected from a group consisting of one or more of thefollowing: a cardiac rhythm management device, a defibrillator, a druginfusion system, an ultrafiltration system, an oral drug dose managementdevice, and a diet and activity management device. In anotherembodiment, controlling a therapy is performed automatically. In yetanother embodiment, controlling a therapy comprises providinginstructions to a recipient, which can be the medical patient.

In one embodiment, the indicator is selected from a group consisting ofone or more of the following: an a-wave, a v-wave, an x-descent, ay-descent, a temperature, a pulmonary artery waveform, a right atrialpressure waveform, a left atrial pressure waveform, an echocardiogramwaveform, and an IEGM waveform. In yet another embodiment, thehistorical pressure data and the historical indicator data are from thepatient.

In other embodiments, hemodynamic waveform analysis systems and devicesare provided to perform any of the methods described herein. In oneembodiment, a hemodynamic waveform analysis device includes animplantable housing, an implantable lead in electrical communicationwith the housing, and a sensor in communication with the lead. Thesensor can include a pressure sensor, an electrical sensor, such as anelectrode, a thermometer, an accelerometer, or a flow rate sensor.

In one embodiment, the housing includes a microprocessor, a memory, adata acquisition circuit, and a telemetry circuit. In anotherembodiment, the housing further includes the circuitry of a pacemaker.

In one embodiment, the microprocessor monitors pressure within a medicalpatient. In one embodiment, the microprocessor is configured to measurean actual pressure in a medical patient in a first time period; measurean indicator of the actual pressure in the first time period, whereinthe indicator is derived from an electrical signal of the patient'sheart; determine a correlative relationship between the actual pressureand the indicator, wherein both the actual pressure and the indicatorare obtained in the first time period; measure the indicator in a secondtime period; and determine the actual pressure in the second time periodbased on the correlative relationship obtained in the first time periodand the indicator obtained in the second time period.

In one embodiment, a method of monitoring pressure within a medicalpatient includes measuring a feature-derived parameter of a pressure;providing an indication of the pressure based at least in part upon arelationship and the feature-derived parameter. The feature-derivedparameter can be selected from the group consisting of: an a-wave, av-wave, an x-descent, a y-descent, a temperature, a pulmonary arterywaveform, a right atrial pressure waveform, a left atrial pressurewaveform, an echocardiogram waveform, and an IEGM waveform.

In some embodiments, the relationship is based at least in part on anamplitude of the feature-derived parameter, and in other embodiments,the relationship is based at least in part on a timing of thefeature-derived parameter. The relationship can include an artificialneural network.

In another embodiment of the present invention, a method of calibratinga pressure monitoring system includes: measuring a pressure with apressure monitoring system; measuring a feature-derived parameter of thepressure; and calibrating the pressure monitoring system based at leastpartially upon a relationship between the pressure and thefeature-derived parameter of the pressure. In one embodiment, thepressure is a left atrial pressure of a medical patient.

In one embodiment, a system for remote calibration of an implantablemedical device includes: a pressure sensor implanted within a medicalpatient that provides pressure signals; and a processor that analyzespressure waveforms to determine a calibration based upon a relationshipbetween the pressure signals and a feature-derived parameter ofpressure. The feature-derived parameter can be selected from the groupconsisting of: an a-wave, a v-wave, an x-descent, a y-descent, atemperature, a pulmonary artery waveform, a right atrial pressurewaveform, a left atrial pressure waveform, an echocardiogram waveform,and an IEGM waveform.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of data acquired from an implanted pressure monitoringdevice in a patient with congestive heart failure;

FIG. 2 is a graph of the correlation between mean LAP and the y-descent;

FIG. 3 is a graph of left atrial pressure data over about ten weeks fromthe same patient;

FIG. 4 is a graph of the difference between each LAP and f-LAP readingover the ten-week period post implantation;

FIG. 5 is a graph of mean LAP post implantation;

FIGS. 6A-6C list examples of signals that may be interpreted tofacilitate diagnosis, prevention, and treatment of cardiovasculardisease; and

FIG. 7 is a table of cardiac and non-cardiac diagnostic states derivablefrom hemodynamic measurements.

DETAILED DESCRIPTION

In one embodiment, waveforms, including those indicative of varioushemodynamic values and parameters, are used with therapeutic devices ascalibration-independent parameters for diagnostic and surrogatemeasurements of left atrial pressure (“LAP”). These waveforms can beuseful in driving therapy. In other embodiments, devices using waveformfeature-derived parameters that are indicative of left atrial pressureand which do not include patient signaling elements drive therapy.

In some embodiments, measurements of LAP are provided based on relativemeasurements of waveform feature amplitude or timing, and do not use anexternal atmospheric pressure reference. In other embodiments,measurements of LAP are provided based on relative measurements ofwaveform feature amplitude or timing, and do not calibrate a pressuresensor zero offset.

In one embodiment, waveform feature-derived parameters or indicators ofLAP (“f-LAP”) are used by an implantable system to drive or optimizetherapy without an external pressure reference. Therapies that can bebeneficially controlled include, but are not limited to: cardiac rhythmmanagement, defibrillators, drug infusion systems, ultrafiltration, oraldrug dose management, diet and activity management. In some embodiments,therapy is controlled automatically. In other embodiments, therapy iscontrolled by providing instructions to a patient, a doctor, a medicalprofessional, or other caregiver.

The term “derived,” is a broad term that is intended to have itsordinary meaning. In addition, the term “derived,” is intended to meanbased upon, related to, comes from, or is otherwise produced as a resultof.

In one embodiment, a system for remote non-invasive calibration ofimplantable medical devices analyzes uploaded pressure waveforms todetermine sensor calibration. In another embodiment, a system remotelymonitors implantable medical devices by analyzing uploaded pressurewaveforms to detect either a change in a physiological condition or amiscalibration of a sensor. In yet another embodiment, a system analyzesmultiple features of an LAP waveform to provide a signal indicative ofthe condition of a heart. In one embodiment the system also provides anoffset calibration of a pressure sensor. In one embodiment the systemdoes not utilize or require an external pressure reference.

In another embodiment, a classifier, such as an artificial neuralnetwork, classifies the condition of a heart based at least in part onthe LAP waveform.

FIG. 1 shows data acquired from an implanted pressure monitoring device,such as a HeartPOD™, in a patient with congestive heart failure (“CHF”).Simultaneous intra-cardiac electrogram (“IEGM”) and LAP pressurewaveforms are shown at four different levels of mean LAP, from 4 to 34mmHg. Changes in features of the pressure waveform are correlated withthe mean pressure, demonstrating that such feature changes areindicative of LAP. There is also a linear relationship between theheight of the y-descent and the mean LAP. This relationship isillustrated in FIG. 2 below.

Pressure waveforms are measured in any of a variety of ways. Forexample, pressure waveforms can be generated from pressure transducersthat are temporarily positioned or permanently implanted within thebody. In some embodiments, pressure is measured with a piezoelectricsensor. In other embodiments, pressure is measured with a pressuretransducer that is implanted within a patient's heart. Examples ofpressure measuring devices are taught in U.S. application Ser. No.10/698,031, filed Oct. 29, 2003, Ser. No. 10/127,227, filed Apr. 19,2002, Ser. No. 10/438,300, filed May 13, 2003, Ser. No. 11/116,138,filed Apr. 27, 2005, and Ser. No. 11/015,336, filed Dec. 17, 2004, whichare all incorporated by reference herein.

FIG. 2 shows data acquired over the first two weeks after implantationof a left atrial pressure sensor in a patient with congestive heartfailure. About four times per day, a 15-second recording of the LAPwaveform was acquired with a sampling rate of 200 Hz. For each acquiredwaveform, the mean left atrial pressure is plotted versus the averageheight of the y-descent, demonstrating the high degree of correlationbetween mean of the pressure waveform and the height of one of itsfeatures. This high correlation is utilized in several embodiments ofthe current invention. Data can be acquired during any interval, such asminutes, hours, days, weeks, months, and/or years, or a combinationthereof.

In one embodiment, the formula for the mean-LAP vs. y-descent regressionline is used to compute an estimate of left atrial pressure from ameasurement of the y-descent. It will be clear to one skilled in the artthat, unlike the mean LAP, the y-descent can be measured withoutreference to the external atmospheric pressure. Thus, an index of leftatrial pressure based on a relative measurement of a waveform feature,such as the height of the y-descent, has the advantage that it does notrequire measurement of an external atmospheric pressure reference.

Another advantage of an index of left atrial pressure based on arelative measurement of waveform features is that such measurements areindependent of sensor offset calibration. Thus, feature-based relativemeasurements indicative of left atrial pressure are advantageouslyunaffected by sensor offset drift. A method and device for measuringleft atrial pressure that does not require an atmospheric pressurereference and which is unaffected by pressure sensor drift isparticularly advantageous for a permanently implantable pressuremeasurement system.

In one embodiment of the present invention, a feature-based index of LAPis used as a substitute for mean LAP to diagnose and drive therapy forworsening congestive heart failure. In another embodiment, afeature-based index of LAP is used together with a direct measurement ofmean LAP. In one embodiment, the relationship between LAP and f-LAP isused to detect a change in the absolute calibration of the sensor.

The relationship between LAP and the feature-based index (or indicator)of LAP is sometimes referred to as a “correlative relationship,” whichis a broad term, intended to have its ordinary meaning. In addition, acorrelative relationship can include any relationship, such as aformula, a graph, a curve, a linear relationship, a non-linearrelationship, a neural network, a classifier, or any other relationshipthat provides a value of one variable when presented with the value ofanother variable.

In one embodiment, the difference between LAP and f-LAP is averaged overan interval to determine an offset calibration for the direct LAPmeasurements. In one embodiment, sudden changes in the relationshipbetween f-LAP and LAP is detected and analyzed. In one embodiment, asudden change in this relationship is an indication of a change in aphysiological condition.

The f-LAP is fundamentally different from other methods and parametersfor estimating LAP. For example, f-LAP depends on relative measures ofthe LAP waveform, and is therefore independent of barometric pressureand sensor offset calibration. Unlike other devices and methods,utilizing f-LAP need not measure pressure relative to a barometricpressure at a specific time in the RV waveform. In addition, in manyembodiments, devices and methods utilizing f-LAP do not utilize anatmospheric pressure reference and a sensor with known zero-pressureoffset.

FIG. 3 shows left atrial pressure data over about 10 weeks from the samepatient. The square symbols are direct measurements of mean LAP obtainedby averaging sensor output for 15 seconds. The diamonds arefeature-based estimates of LAP calculated using a formula that describesthe relationship between the height of the y-descent to LAP. The firsttwo weeks includes the same data as in FIG. 2, which was used todetermine the relationship between LAP and y-descent in the patient. Itis clear that the f-LAP tracks the daily variations in LAP, but thatover time the LAP readings can drift higher relative to the f-LAPreadings.

FIG. 4 shows the difference between each LAP and f-LAP reading over the10 weeks after implant, clearly showing this drift. On 4/28 Valsalvacalibration was performed to determine true LAP. Calibration using aValsalva maneuver is described in detail in U.S. application Ser. No.10/438,309, filed May 13, 2003, and U.S. application Ser. No.10/438,300, filed May 13, 2003, both of which are incorporated byreference herein. The data in FIGS. 3 and 4 both show that thisrecalibration brings LAP and f-LAP back into agreement, demonstratingthat f-LAP accurately predicts mean LAP, and that the difference betweenmean LAP and f-LAP is an accurate measure of LAP sensor offset, whichmay be referred to as an error value.

In one embodiment of the present invention, the difference between LAPand an f-LAP is used to perform sensor calibration. In one suchembodiment, the difference is averaged over a number of measurements orover an interval of time to derive an offset correction. In oneembodiment the interval of time is one week, and an offset correction isapplied to the sensor LAP readings each week. In another embodiment, theinterval is one month, or some other interval that may be chosen by thepatient's physician. In another embodiment, the interval is one week,and a new calibration is applied each day based on the most recentseven-day average difference. Any interval may be selected, such asminutes, hours, days, weeks, months, and/or years, or a combinationthereof.

It is known to those of skill in the art that the relationship between amean cardiac pressure measurement and a waveform feature measurementalso may be affected by physiological changes. A list of examples ofpressure waveform features and corresponding physiological conditions isprovided in FIGS. 6A-6C. Examples of cardiac and non-cardiac diagnosticstates derivable from measurements at the intra-atrial septum are shownin FIG. 7 of the same application.

In several embodiments of the present invention, the systems, devicesand methods employed distinguish between a change in physiologicalcondition and a change in sensor calibration. In one embodiment of thepresent invention, the difference between LAP and an f-LAP is monitoredas described above to derive an offset correction, but the offsetcorrection is not always automatically applied. Instead, a calibrationalert is generated whenever the correction derived from the LAP versusf-LAP difference exceeds a predetermined threshold. In one embodiment,the interval of time is one week and an offset correction is calculatedonce a week. In another embodiment, the interval is one week and theoffset correction is calculated each day based on the most recentseven-day average difference. In another embodiment, the interval is onemonth. Any interval may be selected, such as minutes, hours, days,weeks, months, and/or years, or a combination.

In one embodiment, the alert includes an instruction to the patient toperform a calibration, such as a Valsalva calibration. In oneembodiment, the patient is further instructed to contact his or herphysician if the calibration, such as a Valsalva calibration, does notbring LAP and f-LAP into agreement. In another embodiment, the alertincludes an instruction to the patient to go to the physician's officeto perform a calibration so that the physician can evaluate whether thediscordance between LAP and f-LAP is physiological or sensor drift. Inanother embodiment, two or more LAP feature parameters are analyzed, andthe combination of parameters is used to distinguish between variousphysiological changes (such as those as shown in FIG. 7), and betweenphysiological changes and sensor calibration changes using thisapproach.

Although it is known in the art to recognize various conditions of aheart based on parameters of left atrial pressure (such as theconditions and parameters described in FIG. 7), a more comprehensiveclassification of waveform morphologies according to correspondingphysiological conditions can be achieved by training an artificialneural network (ANN) to do the classification.

As is known in the ANN arts, an ANN generally includes one or moretraining sets of input/output pairs. In one embodiment, the trainingsets include waveforms and known corresponding physiological conditions.The training sets can include the entire waveform, or one or moreparameters derived from the waveform, such as the height of they-descent discussed above. In one embodiment, the ANN is provided ortrained with both an LAP waveform and an IEGM waveform. In anotherembodiment, the ANN is provided or trained with parameters of the IEGM,for example the heart rate and the p-r interval, and the relativepressure values and timing of the maxima and minima in the correspondingLAP waveform.

In one embodiment the ANN output includes a mean LAP and a physiologicalcondition. In one embodiment the physiological condition is one of:tamponade, mitral regurgitation, atrial fibrillation, constrictivepericarditis, as well as all of the conditions listed in FIGS. 6A-6C andFIG. 7.

In one embodiment, an ANN is trained using waveforms and conditions fora single patient and is used to analyze the data from only that patient.Such a restriction improves the accuracy of the ANN for a given numberof training pairs because of the reduced variability in input waveformsfrom a single patient. In another embodiment, an ANN is trained and usedfor classifying waveforms from any patient, or from any of a particulargroup or class of patient.

In one embodiment of the present invention, the analysis required tocalibrate an implanted pressure sensor based on LAP waveform parametersincludes identifying the relevant feature or features within the LAPpressure waveform, measuring the appropriate feature parameter orparameters, and computing the f-LAP from the feature parameters, andsubtracting the f-LAP values from the mean LAP to derive the offsetcalibration. In one embodiment, this analysis is performed by anexternal patient advisory module (“PAM”).

In another embodiment, the analysis is performed by a computer, which isparticularly advantageous when the algorithms for identification offeatures and/or measurement of parameters are complex. In oneembodiment, waveforms stored in the PAM are uploaded to a computer,which can be located in the patient's home. If the calculated offsetcalibration differs from the current calibration, in one embodiment thecomputer downloads the new calibration to the PAM. In another embodimentthe patient is notified to calibrate, and the patient may be instructedto contact the physician, or the patient may be instructed to perform aValsalva calibration as described above.

In another embodiment, the computer is located in the doctor's office,and waveforms from the patient's PAM are uploaded, and a new calibrationis downloaded, such as during an office visit. In another embodiment,the computer performing the waveform analysis may be remote from thepatient, and the patient uploads data from the PAM telemetrically, forexample via a telephone modem or an internet connection, as is wellknown to those skilled in the art. In such embodiments, the remotecomputer can be located at the doctor's office, or at a central siteconfigured to receive and process calibration data from multiplepatients' implanted devices.

In one embodiment, the uploaded data includes raw LAP waveform data,simultaneously acquired IEGM waveforms, and current calibration factors.The remote computer performs f-LAP analysis to determine a newcalibration. If the PAM requires a calibration change, the newcalibration based on the parameters of the telemetrically uploadedwaveforms may be downloaded telemetrically to the PAM, providing asystem for remote calibration of implanted medical devices. In anotherembodiment, the patient and/or the doctor are notified that are-calibration is required.

1. A system for estimating cardiac pressure within a patient using animplantable medical device, the system comprising: an electrical fieldapplication system operative to apply an electrical field to tissues ofthe patient including cardiac tissues; a measurement unit operative tomeasure a parameter influenced by the electrical field, the parameterbeing affected by cardiac pressure; and a cardiac pressure estimationunit operative to estimate cardiac pressure within the patient byapplying predetermined conversion factors to the measured parameter. 2.The system of claim 1 wherein the electrical field application systemincludes an impedance detection pulse generation system.
 3. The systemof claim 1 wherein the measurement unit includes: an electricalimpedance (Z) detector operative to detect an electrical impedance (Z)signal within the patient along a sensing vector wherein impedance isaffected by cardiac pressure; and a parameter determination unitoperative to derive the parameter from the electrical impedance (Z)signal.
 4. The system of claim 1 further including a heart failuredetection system operative to detect heart failure based on estimatedcardiac pressure.
 5. The system of claim 1 further including animplantable warning device operative to generate a warning upondetection of heart failure.
 6. A system for estimating cardiac pressurewithin a patient using an implantable medical device, the systemcomprising: means for applying an electrical field to tissues of thepatient including cardiac tissues; means for measuring a parameterinfluenced by the electrical field, the parameter being affected bycardiac pressure; and means for estimating cardiac pressure within thepatient by applying predetermined conversion factors to the measuredparameter.